In known foldable wheel chairs available on the market the seat and back are usually manufactured from a flexible strong textile material such as canvas, so that the chair is folded around the vertical symmetry plane parallel to the wheels. In such chairs, it is often a disadvantage to the user that the flexible material of the seat and the back does not provide sufficient support. Moreover, even in its folded condition such a chair will normally occupy much space in the vertical direction, since the back may normally not be folded relative to the wheels.
Such a relatively great demand of space is also a disadvantage of a known chair disclosed in Danish Design Application No. 914/1977 which has been published in the Danish Design Journal (Registreringstidende for Monstre) of 15th Feb. 1978. Contrary to most wheel chairs, this known chair is provided with quite small wheels and has in the same manner as ordinary folding chairs a support structure, the mutually pivotally connected parts of which may be pivoted around a pivot axis parallel to the wheel shafts whereby a possibility is provided to use a seat and a back of a formstable, non-flexible material.
Another wheel chair for retarded persons, which chair is foldable by pivotal movement of mutually pivotally connected supporting parts around axes parallel to a wheel shaft, has been disclosed in Danish Design Application No. 394/1977, which has been published in the Danish Design Journal of 1st Dec. 1977. The supporting parts for the seat comprise in this case several mutually pivotally connected link rods which complicate the manufacture and make it more expensive and, furthermore, make folding of the chair and rearrangement of it from a folded condition to the normal operative state more difficult.
From Swedish published patent specification No. 373,325, a foldable baby carriage is known, in which a front wheel supporting frame is pivotally connected with a back wheel supporting frame around an axis parallel to the the wheel axes, whereas the back wheel supporting frame is pivotally connected around an axis displaced in parallel relationship thereto with a U-shaped frame structure forming a handle and being provided in the lower free end in each side with a bent interlocking part engaging a locking bushing on the side pieces of the front wheel supporting frame. The engagement of said interlocking part and said locking bushing, which must be easily movable relative to each other, is released by slightly lifting the handle frame to open the triangular link which in the raised position of the carriage constitute the blocking mechanism, whereby the back wheel supporting frame is allowed to turn against the front wheel supporting frame followed by pivotal movement of the two wheel supporting frames up towards the handle frame. In addition to the fact that the release movement is associated with a considerable risk of unintended folding of the carriage due to the very simple blocking mechanism, the construction requires at least the back to be of a flexible material which due to the reasons mentioned in the foregoing would lead to considerable disadvantages when used in a wheel chair.
In U.S. Pat. No. 3,450,432, a usual folding chair not provided with wheels is disclosed, said chair having a seat and a back of a formstable, non-flexible material. In this case, a limited number of pivotally connected supporting parts, a small demand of space in a folded condition and a simple operation when folding and re-raising the chair, have been obtained in that the seat, the back, a front leg supporting structure and a back leg supporting structure are mutually pivotally connected only by means of two identical pivot assemblies positioned opposite each other at the seatback junction and including a blocking mechanism which is releasable by upwards turning of the seat towards the back. In this known construction, the back is only secured in the normal operative state of the chair relative to the seat by engagement between two opposed shoulder parts on flange portions connected with the seat and the back, respectively, whereby the back is only prevented from turning in a direction to increase the angular separation of the seat and back, whereas it is free to turn in the opposite direction, and no blocking is present between the back and the leg support structures. Thereby, also this construction suffers from a considerable risk of unintendedly folding when a person sitting in the chair leans back heavy enough to turn the seat upwardly beyond a position, in which the pivotal movement of the seat results in folding of the leg support structures. Furthermore, in spite of the intended simplification, reraising of the chair is difficult in that the back must be turned 180.degree. and the seat 270.degree. from the folded condition. Moreover, the two pivot assemblies are not safely synchronized, so that already at a modest heavy or robust manipulation there will be a risk of distortion of the pivot assemblies. Finally, due to the above mentioned movability of the back in the normal operative state, a chiar constructed in this manner will not be suitable for use as a wheel chair.